U.S. Pat. No. 5,483,818 discloses an ion measuring circuit of the type described above. The known circuit comprises an inverting amplifier wired as a differential amplifier, the inverting input of which is connected through a resistor to the low voltage potential side of the secondary winding of an ignition coil also referred to as an ignition transformer having a primary winding and a secondary winding. The differential amplifier has a further non-inverting input connected to a biasing voltage of about 40 V. The inverting amplification characteristic of the differential amplifier is achieved by connecting a feedback resistor in parallel between the output of the amplifier and the inverting input thereof. The output of the differential amplifier is simultaneously connected to a signal evaluating threshold circuit. Furthermore, the low potential end of the secondary winding is connected through two series connected Zener diodes to ground potential. The connecting point between the two serially connected Zener diodes is connected to a further inverting amplifier which controls this connecting point in such a way that during an ion current measuring operation any leakage currents are avoided in order to produce accurate ion current representing signals which are not falsified by any leakage currents. The second inverting amplifier is wired or constructed in the same way as the inverting amplifier directly connected to the low voltage end of the secondary winding. In other words, the output of the second amplifier is also connected through a resistor to the inverting input of the second amplifier. The non-inverting input of the second amplifier is connected to the same biasing voltage of about 40 V as the first amplifier.
The circuit arrangement of the above mentioned U.S. Patent is not cost efficient since it requires two inverting differential amplifiers for avoiding ion current measurement errors that may otherwise be caused by the leakage currents of the two Zener diodes. Thus, there is room for improvement.